A new study, to be published in a forthcoming issue of Alcoholism: Clinical and Experimental Research, found that using diet soda as a mixer can dramatically increase breath-alcohol content (BAC) without increasing your awareness of being impaired.

The study, which compared Smirnoff Red Label plus Squirt (a lemon-lime soda with no caffeine) with the vodka with Diet Squirt, found that the diet cocktail increased BAC by 18%. That’s almost as much as having an additional standard drink and was enough to tip people from being under the legal limit for driving to being unsafe to drive.

“One of the key things we found was that even though BAC peaked 18% higher in the diet condition, [participants] didn’t feel any more intoxicated and they didn’t feel any different as to how willing they were to drive a car,” says the study’s lead author Cecile Marczinski, who is an assistant professor of psychology at Northern Kentucky University.

The study involved 16 young men and women who consider themselves to be social drinkers. Over the course of three different drinking sessions, they were given either a placebo drink, a diet Squirt with vodka, or a full-sugar Squirt with vodka. Researchers decided to test a noncaffeinated diet drink because although caffeine doesn’t affect breath-detectable levels of alcohol, it does affect whether people think they are intoxicated so would have made the results more difficult to interpret. The dose of vodka was calibrated to each participant’s weight and gender to put them close to the .08 legal limit for drinking, which is the equivalent of drinking four beers in an hour.

After drinking the cocktail they were assigned, participants were tested for their reaction time and the number of errors they made in a computer task in which they learned a response and then had to inhibit that reaction on cue. They were also asked about their perceptions of how drunk and tired they were and whether they felt they could safely drive. At all times, the amount of alcohol measured in the volunteers’ breath was higher among those whose cocktails included diet drinks. And while the participants never reached the legal limit when they drank sugared soda with alcohol, they exceeded the safe limit for driving within 40 minutes if they had the diet cocktail.

“They are more impaired on the computer task, measured both by reaction time and errors,” Marczinski says of those in the diet-drink group. “You shouldn’t trust your own judgment of impairment. In one case, the subjects were safe to drive and legal, and in the other case, they were not, but they had no idea.”

So why would diet drinks increase intoxication? Previous work found that the stomach reacts to sugary drinks as though they are food, working to digest the calories. Eating solid food while drinking significantly reduces BAC, by 20% to 57%, and sugary drinks seem to act similarly, keeping alcohol in the stomach longer, slowing down its release into the bloodstream.

With diet soda, by contrast, Marczinski says, “The stomach doesn’t recognize that it needs to do anything with that drink, because it has no sugar. It goes right to small intestine where the most alcohol is absorbed into the bloodstream.”

Marczinski recognizes that the results may prompt some to turn to diet drinks for a quicker and cheaper buzz, but, she says, the connection is important to recognize so bartenders and party hosts can look out for those who keep ordering diet-mixed drinks. “I think people need to be aware of this.”

The diet soda may simply increase the conversion of blood alcohol to breath alcohol. It does not necessarily increase the uptake of alcohol into the blood. A proper study to test this would test blood alcohol as well as breath alcohol, to account for the potentially confounding variable of blood alcohol expiration rate.